The present invention relates generally to transfer cases for use in four-wheel drive vehicles. More particularly, the present invention relates to a transfer case having a synchronized range shift mechanism for on-the-move shifting between high-range and low-range drive modes.
The drivetrain in many light-duty and sport-utility vehicles includes a transfer case for transmitting drive torque to all four of the wheels, thereby establishing a four-wheel drive mode of operation. To accommodate differing road surfaces and conditions, some transfer cases are equipped with a gear reduction unit which can be selectively shifted by the vehicle operator to establish four-wheel high-range and low-range drive modes. In many instances, however, the vehicle must be stopped before the transfer case can be shifted between its four-wheel high-range and low-range drive modes. Unfortunately, the need to stop the vehicle prior to such range shifting is inconvenient, particularly upon encountering road conditions or surface terrains where continuation of the vehicle's rolling momentum would assist in overcoming the conditions encountered. As such, some gear reduction units are adapted to permit the vehicle operator to shift from the four-wheel low-range drive mode into the four-wheel high-range drive mode without stopping the vehicle. For example, U.S. Pat. No. 5,054,335 discloses a transfer case equipped with a synchronized range shift arrangement for on-the-move shifting of a layshaft-type gear reduction unit. Alternatively, commonly-owned U.S. Pat. No. 5,346,442 discloses a transfer case having a synchronized range shift arrangement for on-the-move shifting of a planetary-type gear reduction unit. Finally, U.S. Pat. Nos. 4,569,252, 5,651,749, 5,655,986, 5,697,861 and 5,704,867 disclose transfer cases equipped with planetary-type gear reduction units which permit synchronized shifting between the high-range and low-range drive modes. Thus, a recognized need exists to develop synchronized range shift systems for use in four-wheel drive transfer cases that advance the current technology.